1. Field of the Invention
The invention relates to production of a microcellular foam.
2. Description of the Prior Art
Several processes exist for preparation of polymer foams and ceramic foams. These foam preparation techniques include slurry, chemical vapor deposition (CVD) aerogel, fiber co-pressing and binding, vapor phase reaction bonding, preceramic foam pyrolysis, phase separation, and replication processes.
According to the slurry process, a polymer foam is coated with a ceramic slurry. In subsequent processing steps, the slurry-coated polymer foam is exposed to elevated temperatures so that the slurry is sintered and the polymer burned away resulting in a foam having the composition of the ceramic and the microstructure of the polymer foam. The slurry process is relatively economical; however, the resulting ceramic foams are relatively weak and have a coarse-celled microstructure.
Chemical vapor deposition (CVD) processes also begin with a template foam, such as a carbon foam, which is infiltrated with the chemical vapor. The template foam serves as a substrate on which the chemical vapor decomposes to deposit the desired ceramic. The template foam can be left in place or removed by a further processing step such as pyrolysis. Typically, CVD processes are relatively expensive.
Aerogel processes produce foams directly from a sol-gel foam and, hence, are inherently limited to the sol-gel chemistries available and to the cellular microstructures they form.
Vapor-phase reaction bonding processes, such as those described in German Patent DE 2517380, require fabrication of an appropriate microstructure foam of one of the reactive elements, typically of an element such as silicon or carbon.
Preceramic polymers-based processes, in which preceramic polymers are pyrolyzed to form a ceramic foam, by nucleation and growth of gas bubbles within the preceramic polymer are described by Wynne et al., Ann. Rev. Mater. Sci., 14, 1984, 297-334 and Baney et al., in Conference on Emergent Process Methods for High-Technology Ceramics, Materials Science Research Series Vol. 17, Davis et al. Eds., North Carolina State Univ., 1982, Plenum Press, New York, 253-262. Such direct production results in foams characterized by microstructures of wide and poorly controlled cell size distribution and cell volume fraction.
All of these processes are either relatively expensive, or produce coarse or irregular foams. Thus, there exists a need for a relatively inexpensive method for producing foams, particularly, ceramic foams, such as ceramic compositions based on silicon, carbon, nitrogen, boron or titanium, having a fine, controlled, regular and open celled microstructure with no trace of a starting template foam.